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    Subject[PATCH 22/73] union-mount: Add two superblock fields for union mounts [ver #2]
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    From: Valerie Aurora <vaurora@redhat.com>

    Add two fields to struct super_block to support union mounts:

    (*) s_union_lower_mnts

    A pointer to a cloned vfsmount tree of all the lower (read-only) mounts
    unioned with the topmost (read-write) vfsmount. These mounts may have
    submounts which will also be unioned; hence we copy the entire vfsmount
    tree, not just the root vfsmounts.

    (*) s_union_count

    The number of lower mounts unioned at the root of the file system. This
    count is the maximum number of directories that will ever be unioned with
    a single directory. We use it to allocate a union stack of the correct
    size for each directory.

    Original-author: Valerie Aurora <vaurora@redhat.com>
    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    include/linux/fs.h | 10 ++++++++++
    1 files changed, 10 insertions(+), 0 deletions(-)

    diff --git a/include/linux/fs.h b/include/linux/fs.h
    index a014f0f..f19772c 100644
    --- a/include/linux/fs.h
    +++ b/include/linux/fs.h
    @@ -1509,6 +1509,16 @@ struct super_block {
    * free_vfsmnt() if MNT_HARD_READONLY is set.
    */
    int s_hard_readonly_users;
    +
    + /* Root of the private cloned vfsmount tree of the read-only
    + * mounts in this union (set in topmost vfsmount only)
    + */
    + struct vfsmount *s_union_lower_mnts;
    +
    + /* Number of layers in this union, not counting the topmost or
    + * submounts.
    + */
    + unsigned int s_union_count;
    };

    /* superblock cache pruning functions */


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